This invention relates to apparatuses and methods for reducing jet engine noise, and more particularly to a high frequency resonance tube excitation system for injecting a plurality of high frequency, pulsed air streams within or adjacent to an exhaust nozzle of a jet engine, or a cavity of an aircraft, to modify the shear layer exhaust airflow exiting from the exhaust nozzle or moving over the cavity during operation of the jet engine.
Reducing the noise emitted from jet aircraft engines, and more particularly jet aircraft engines associated with military aircraft, has gained increasing importance in recent years. The high levels of jet engine noise can contribute to wear and tear on the various panels used with an aircraft. In certain extreme instances, sonic fatigue caused by excessive jet engine noise may result in repairs being required to various panels of an aircraft. With military aircraft, the sonic fatigue generated by present day jet engines can cause sonic fatigue of various components associated with a weapons bay of the aircraft, as well as to wheel wells and other aircraft cavities. The exhaust airflow from today""s jet engines can also cause ground erosion and present other environmental concerns as a result of extremely high noise levels.
With military aircraft, the high noise levels associated with the operation of present day jet aircraft engines is a factor that must be considered in the safe release and accurate delivery of weapons carried by the aircraft. The imposition of low observable requirements have recently emphasized the carriage of weapons in internal weapons bays of military aircraft. Experience has shown that an open cavity at moderately high speeds can have a profound effect on internally carried weapons, their suspension equipment, separation characteristics and the structural loads on the aircraft itself.
One of the problems associated with internal cavities is acoustic resonance. Acoustic resonance causes high pressure and high frequency fluctuating acoustic loads in and near the weapons bay. These fluctuating pressure loads are high enough to very quickly fatigue metal parts and damage sensitive electronics on xe2x80x9csmartxe2x80x9d weapons.
Managing the acoustic field generated by present day vertical take-off and landing (VTOL) aircraft is also of high importance. Significantly reducing the acoustic field would allow the weight of military aircraft to be reduced by eliminating the need for reinforced panels capable of withstanding the sonic fatigue caused by the acoustic environment of the impingement airflow. Significantly reducing the acoustic field would also alleviate the ground erosion caused by the lift jet system of VTOL aircraft that results in spalling of concrete, which in turn increases the risk of foreign object damage (FOD) to the engines of the aircraft. Reducing the acoustic field would also help to eliminate damage to stores caused due to internal buffeting by cavity resonance phenomena when the weapons bay doors of the aircraft are open during flight. Finally, reducing the acoustic field would significantly extend the permissible exposure time of personnel working in the vicinity of aircraft operations.
Active flow control has been touted as an attractive technique for attaining suppressed acoustic levels in military aircraft weapons bays across a wide operating envelope. Interest in active flow control methods centers around the promise of adjustable, optimal suppression for changing flight conditions. Acoustic suppression utilizing active flow control is usually achieved by seeding the unstable free shear layer traversing the bay with small amplitude disturbances (vortical, acoustic or otherwise). These xe2x80x9cplantedxe2x80x9d disturbances grow and, in many successful cases, compete with and overwhelm the naturally occurring disturbances which would otherwise lead to potentially dangerous acoustic resonance.
Previous attempts to apply active flow control to high speed cavities in a laboratory environment have focused on perturbing the shear layer traversing the bay at frequencies near the dominant Rossiter modes. This type of forcing will be referred to as xe2x80x9clow frequencyxe2x80x9d forcing. These low frequency (LF) active flow control attempts have met with some moderate success, but in most cases there are still, significant, energetic tones remaining after suppression which could be further reduced. Overall suppression using low frequency suppression techniques is typically on the order of about 5 dB to 10 dB in noise reduction. However, it must be kept in mind that this is the same level of performance that might be expected from a xe2x80x9cno-moving-partsxe2x80x9d, low complexity, leading edge spoiler found on today""s military strike aircraft with weapons bays.
Yet another inherent difficulty with low frequency suppression techniques is that of suppressing more than a single acoustic mode at a time. In LF flow control, a choice is often required between either accepting marginal overall suppression levels (a compromise based on suppressing one tone while augmenting others) or implementing a strategy which dedicates a separate controller to each acoustic mode to be suppressed.
It has also been known for some time that steady, energetic mass injection at the leading edge of a high speed cavity results in a dramatic reduction in acoustic resonance tone levels. This is due to the fact that the shear layer traversing across the bay (i.e., cavity) is being literally xe2x80x9cblown offxe2x80x9d, and the impingement of the shear layer on the downstream wall of the cavity is being significantly reduced or eliminated. This type of control leads to simultaneous suppression of all resonant tones in the cavity SPL spectra, but at a rather severe cost in terms of injected mass flow rate.
In view of the foregoing, it is a principal object of the present invention to provide an active noise suppression apparatus and method for use with an aircraft for simultaneously suppressing multiple acoustic modes of the noise produced by a jet engine of the aircraft across a wide frequency spectrum.
It is still a further object of the present invention to provide an apparatus and method for actively, simultaneously suppressing multiple acoustic modes of noise generated from a jet engine in the vicinity of various cavities of an aircraft.
The above and other objects are accomplished by an apparatus and method using active flow control techniques for reducing jet engine noise across a wide frequency spectrum. In one preferred embodiment the invention comprises a series of resonance elements which are placed adjacent to one another about the periphery of an exhaust nozzle of a jet engine. Each resonance element has an inlet port and an outlet port and an interior area in communication with the inlet port and the outlet port. A pressurized air chamber supplies a pressurized airflow through the inlet of each resonance element. As the air flows through the inlet it produces a supersonic jet airflow within the resonance element. This supersonic jet airflow produces an oscillating, high frequency airflow that is dependent on the internal volume of the resonance element. This high frequency, oscillating airflow produces a high frequency, pulsed, excitation air stream through the outlet and into the interior area, or adjacent an edge, of the jet engine exhaust nozzle. The high frequency, pulsed, excitation airstream induces modifications into the shear layer of the exhaust airflow exiting from the exhaust nozzle which result in a major reduction in the noise and buffeting generated by the developing shear layer.
In one preferred embodiment air is introduced at a pressure of about 100 psi or greater into the pressurized chamber. The resonance elements are spaced apart from one another around at least a portion of the circumference of the exhaust nozzle, and preferably around the entire periphery of the exhaust nozzle. The method and apparatus of the present invention is capable of reducing noise levels of jet engines by as much as 10 dB overall sound pressure level (OASPL), and by 20 dB or more for spectral peaks associated with STVOL aircraft in a landing environment. Noise reduction benefits for commercial aircraft are also achieved with the active flow control system of the present invention.